1. Field of the Invention
This invention relates to the stabilization of polyester amides prepared by the melt condensation of 2,2-dimethylpropanediol, cycloaliphatic and/or aliphatic, diprimary diamines and dicarboxylic acids and/or .omega.-aminocarboxylic acids or lactams. More particularly, the present invention relates to the stabilization of such polyester amides against the formation of a skin. This invention also relates to the stabilization of such polyester amides against discoloration.
2. Discussion of the Prior Art
In parent application Ser. No. 380,423 there is described a process for preparing polyester amides which are not readily discolored and are of a clear or semi-clear nature by heating in the melt an aliphatic or cycloaliphatic diamine, 2,2-propanediol and an aliphatic or aromatic dicarboxylic acid and/or a cycloaliphatic or aliphatic .omega.-aminocarboxylic acid or precursor thereof. The polyester amides so obtained have good color stability and are generally clear or semi-clear in nature. Unfortunately, it has been found that some polyester amides obtained by such polymerization do not have the desired melt stability for application as thermoplastic adhesives especially when the polyester amide is mainly or exclusively of hexamethylene diammonium adipate ("AH salt"). This is especially the case when the product is held in the melted state in open, heated reservoirs from which it is pumped as required to some kind of application apparatus such as injection nozzles, applicator rolls, cylinders, spray guns and the like. This seriously impairs the ability of the polyester amide to be used as a thermoplastic adhesive. In such cases where the polyester amide is maintained in the melted state in an open, heated reservoir a skin of infusible and insoluble matter forms to a greater or lesser extent on the surface of the melt, depending on the length of exposure to the air.
When fresh polyester amide is added to the reservoir, this skin is easily broken up and migrates in the form of solid or slimy particles into the interior of the melt and from there into the lines of the applicator devices and on to the latter, thereby disturbing the uniform application of the melt or even completely interrupting the flow of material. This necessitates difficult and expensive cleaning operations. For this reason it has become desirable to prevent the formation of skin under the above-described conditions or at least to retard such skin formation to such an extent that it will cease to be a problem.
Surprisingly, the undesirable skin formation does not occur in the polyesters of Ser. No. 380,423, even in the absence of any stabilizers, if the amide groups contained in them originate from an aminocarboxylic acid or lactam thereof which does not give rise to formation of AH salt. The sole or predominant use of AH salt in the preparation of polyamides, however, is often advantageous because the polyester amides thereby obtained crystallize substantially faster upon the cooling of the melt than those which contain amino acids condensed into their structure instead of AH salt, especially epsilon-aminocapronic acid or caprolactam. The greater speed of crystallization results in a more rapid setting of the adhesive compositions. This more rapid setting in turn permits the performance of a greater number of adhesion operations per unit time, which is often desirable or necessary for the purpose of increasing output. Consequently, the stabilization of polyester amides containing AH salt condensed into their structure so as to prevent the formation of skin when the polyester is heated and exposed to air is an economically important objective.
Another important consideration in the stabilization of polyester amides containing AH salt condensed into their structure is the requirement that the products be as light in color as possible and remain so. This requirement is established because the adhesive when squeezed out of the joint formed when objects are glued together should not be so visible as to noticeably and visibly stain the area of the joint.
It has, therefore, become a desirable to provide a stabilizer composition which stabilizes a polyester amide, especially one containing AH salt within the structure, against the formation of a film due to the adverse effect of air when the polyester is heated. It is also desirable to provide such a stabilizer composition which will impart rare light stability to the polyester amide.